Pullups are a super exercise for training the latissimus dorsi, the broad muscle that lies at the back of the ribs. This total body move doesn't stop there, though. Pullups use most of the major muscles of your upper body, including the posterior deltoid. Always consult your doctor before beginning this or any exercise regimen.
The posterior deltoids, which make up the back of the shoulder, assist the lats as you pull up over the bar. Teres major is another muscle that is part of the scapulohumeral muscle group that, like the deltoid, act on and stabilize the glenohumeral joint of the shoulder during the pullup. Several other muscles on the backside of the body, including the rhomboids and trapezius muscles also assist. Your arms, particularly the forearms and biceps, chest, pectoralis minor and a small muscle around the sides of the neck called the levator scapulae, are worked during a pullup, too.